The present invention relates to a gas sensor (such as oxygen sensor, NOx sensor, HC sensor etc.) for detecting the concentration of a specific gas component in a gas under measurement.
Hereinafter, the term “front” refers to a gas sensing side with respect to an axial direction of a gas sensor; and the term “rear” refers to a side opposite the front side.
Japanese Laid-Open Patent Publication No. 2012-189579 discloses one type of gas sensor (e.g. oxygen gas sensor) that includes an elongated plate-shaped or rod-shaped sensor element having, at a front end part thereof, a detection portion constituted by an oxygen ion conducting solid electrolyte material and a pair of detection electrodes. In this type of gas sensor, the sensor element is held in position by a holder within a cylindrical metal shell (metal housing). The holder is formed of an insulating material (such as ceramic material e.g. alumina) with an axial through hole so that the sensor element is inserted through the axial though hole of the holder with the front end part (detection portion) of the sensor element protruding toward the front from a front end of the holder. The metal shell is equipped with a thread portion so that the gas sensor is mounted to e.g. an exhaust pipe of an internal combustion engine by screwing the thread portion into a mounting hole of the exhaust gas. In a state where the gas sensor is mounted to the engine exhaust pipe, the detection portion of the sensor element is situated inside of the exhaust pipe and exposed to an exhaust gas (as a gas under measurement) so as to generate an electric signal responsive to the concentration of a specific gas component (e.g. oxygen) in the exhaust gas. Electrode terminals are formed on a rear end part of the sensor element and connected to an ECU (engine control unit; as an external device) through leads so that the electric signal of the sensor element is outputted to the ECU for air-fuel ratio control etc.
In order to cover the front end part of the sensor element and protect the detection portion from water contained in the exhaust gas, a protector (protection cover) having a plurality of vent holes is attached to a front end of the metal shell. In addition, a protection layer of porous material (e.g. spinel or alumina) is commonly formed on a predetermined surface region from the front end of the sensor element toward the rear in order to prevent the sensor element from deteriorating in detection performance due to the adhesion of a foreign substance contained in the exhaust gas or becoming broken due to the adhesion of water contained in the exhaust gas. The formation of such a porous protection layer is particularly effective as a measure against the breakage of the sensor element caused due to thermal impact by the adhesion of water in the case where a heater is provided on the sensor element for rapid heating and activation of the solid electrolyte material.
Herein, a noble metal material (e.g. platinum or alloy thereof) is used for various conductors (conduction layers) such as detection electrodes, heater energization electrodes etc. in the above-mentioned sensor element. The amount of use of the noble metal material increases with the axial length of the sensor element, which results in a cost increase. It is demanded to reduce the axial length of the sensor element for cost reduction. In recent years, there has been a demand to reduce the axial length or size of the sensor element for the purpose of not only decreasing the amount of use of the noble metal material for cost reduction but also decreasing the power consumption of the sensor element for energy conservation.
For the length reduction of the sensor element, it is conceivable to decrease the amount (length) by which the rear end part of the sensor element protrudes toward the rear from the holder. As mentioned above, the rear end part of the sensor element is a place where the electrode terminals are formed for electrical connection to front ends of the leads by crimp contacts. It is thus necessary to secure a predetermined length of the rear end part of the sensor element in order to improve the reliability of the electrical connection. It is also necessary, in view of the fact that the detection portion of the sensor element reaches a high activation temperature, to locate the electrode terminals apart (away) from the detection portion in order to avoid the electrical connection from being influenced by heat from the detection portion. For these reasons, there is a limit to the decrease of the protrusion amount (length) of the rear end part of the sensor element.